Acrylic acid (AA) is currently made commercially by the two-step catalytic oxidation of propylene. More recent, but not-yet-commercial technology exists for its manufacture by the catalytic oxidation of propane. Propylene is a petroleum derivative, and its price reflects the growing scarcity and rising price of oil. Propane, derived from oil or natural gas liquids, makes a convenient fuel, and its price has risen as it has been used as a substitute for petroleum fuels in energy production. Both propylene and propane are non-renewable resources. It is desirable to find a renewable feedstock for the manufacture of acrylic acid.
A diesel fuel can be made from renewable materials by transesterification of natural fats and oils. Transesterification with methanol yields fatty acid methyl esters, also known as FAME or biodiesel, and glycerol. The amount of glycerol produced in this way has already outstripped demand, and the amount of this “waste” glycerol is projected to increase. It is desirable to find a use for this glycerol. Glycerol is also available as a by-product of hydrolysis of various oils and fats, as well as from waste fluids in soap production.
The dehydration of glycerol to acrolein, in either vapor or liquid phase, is well-known. A variety of acids have been used to catalyze this reaction, including mineral acids, potassium bisulfate, zeolites, Nafion composites, and modified zirconias. See, e.g., U.S. Pat. No. 2,042,224, U.S. Pat. No. 2,558,520, U.S. Pat. No. 5,387,720 and U.S. Patent Publication US2006/092272. In processes for liquid phase dehydration of glycerol, the catalyst can be suspended in an organic liquid, such as an alkane or a mixture of alkanes (e.g., hexadecane or paraffin wax). In such liquid phase processes, some of the solvent distills and is separated from the water into the acrolein product, necessitating separation and recycle of the solvent. Not only does this complicate the process and add cost to it, but since acrolein has some solubility in the solvent, some acrolein is lost in this process. Also, it is known that acrolein is highly reactive at elevated temperatures such as those used during dehydration of glycerol and, therefore, prolonged exposure to the heat of the reaction mixture will result in losses of the desired acrolein product. Thus, prompt removal of the acrolein product from the reaction mixture is important to maximize acrolein yields.
The vapor phase reaction is generally most selective when carried out in the presence of a large quantity of water, e.g., aqueous solutions containing 20% or less by weight glycerol. As the fraction of glycerol in the feed is increased, side reactions forming glycerol ether dimers and oligomers occur with greater frequency, lowering the overall acrolein yield. Dehydration of an aqueous solution of glycerol having a glycerol concentration of only 20% would require a relatively large reactor for a given productivity, increasing both capital and operating expenses. Additionally, it is mentioned in U.S. Pat. No. 5,387,720 that, while dehydration occurs using aqueous glycerol of greater than 40% by weight glycerol, the selectivity of the reaction to acrolein and the service life of the catalyst are appreciably reduced at higher concentrations, and a glycerol concentration of between 10 and 25% by weight glycerol is recommended.
International Patent Application Publication WO 2006/092272 describes the dehydration of 0.1-90% glycerol solution to acrolein followed by the vapor-phase oxidation of acrolein to AA. The dehydration is carried out using catalysts having a Hammett acidity Ho of +2 to −3 for liquid-phase reactions, and −3 to −8.2 for gas-phase. While this patent application discusses liquid phase dehydration of glycerol to acrolein, no mention is made of an organic solvent.
U.S. Pat. No. 2,558,520 provides a process for liquid phase dehydration of glycerol to acrolein over a supported acidic or anhydrous phosphorous-based catalyst, using paraffin (alkane) hydrocarbons as a solvent. It is acknowledged that a small portion of the paraffin solvent distills over with the acrolein product.
Japanese Unexamined Patent Application Publication JP 2006-290815 describes the liquid phase dehydration of glycerol using a solid acid catalyst having a Hammett acidity Ho between +3.3 and −5.6 in a solvent. This application provides working examples using potassium bisulfate as the catalyst, and solvents include alkanes and paraffin wax.
The present invention addresses the aforesaid problems by using a melt of one or more vinyl polymers of suitable molecular weight as the liquid solvent. The reaction proceeds as with alkane or paraffin wax, but without distillation of solvent with the acrolein product. Thus, the aforesaid problems with product separation, solvent recycle, and yield loss are obviated. Also, a more concentrated aqueous glycerol solution (i.e., greater than about 20% by weight glycerol) may be used as the feed to this process.